Sinusoidal marine seismic data acquisition

ABSTRACT

A method of acquiring marine seismic data using an acoustic source to generate an acoustic signal, a portion of which is reflected at one or more subsurface formation interfaces as a seismic signal is described. The method includes sailing a surface vessel along a sail line which lies over a survey area while towing a seismic streamer, the sail line having a sinusoidal configuration defined by an amplitude and a wavelength. The streamer includes a plurality of hydrophones for receiving the reflected portion of the acoustic signal. The method is characterised in that the streamer follows the sinusoidal configuration of the sail line while seismic data is acquired, the streamer having a length at least equal to the distance travelled by the surface vessel as it sails along one full wavelength of the configuration as measured along the sinusoidal sail line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/AU2009/000673, filed on May29, 2009, which claims priority from U.S. patent application Ser. No.12/128,980, filed May 29, 2008, and from Australian Patent ApplicationNo. AU 2008906452, filed on Dec. 15, 2008, the disclosures of which areincorporated herein by reference in their entireties.

FIELD

The present invention relates to the field of marine seismic dataacquisition methods and related processes. The present invention alsorelates to a method and system for undertaking a survey over ageological feature within a survey area. The method and system areparticularly, though not exclusively, suited to undertaking a surveyover geological feature suitable for bearing hydrocarbons.

BACKGROUND

Surveys over geological structures are generally conducted using seismicdata acquisition methods or electromagnetic acquisition methods. Marineseismic exploration investigates and maps the structure and character ofsubsurface geological formations underlying a body of water. Usingconventional acquisition techniques, an ocean-going vessel is used totow one or more acoustic sources and one or more seismic streamer cablesthrough the ocean along predetermined sail lines. A suitable acousticsource is created by the collapsing of an air bubble, and prior artacoustic sources typically comprise compressed air guns for generatingacoustic energy in the water called ‘shots’. The basis of marine seismicdata acquisition methods is the accurate timing of artificiallygenerated pulses of acoustic wave energy that propagate through theocean and are reflected at the interfaces between subsurface formations.These reflected pulses which are referred to as “seismic energy” or“seismic signals” (because of the interaction of the acoustic energywith the geological formation) are detected using transducers calledhydrophones that transform the seismic energy into electromagneticsignals. Each streamer towed behind the vessel typically supportsmultiple hydrophones and the data collected by each hydrophone isrecorded and processed to provide information about the underlyingsubsurface geological features. Using conventional acquisitiontechniques, towing of the streamers is undertaken at a predeterminedspeed and along predefined parallel and linear sail lines to assist withthe collection and processing of the data acquired by the hydrophones.

A portion of the acoustic energy fired from an acoustic source travelsdownwardly through a body of water towards a subsurface geological and aportion thereof is reflected upward from the subsurface geologicalformation as a response signal. This response signal is collected at ahydrophone. The amplitude and the time taken for the response signal tobe received at the hydrophone are indicative to some degree of the depthof subsurface geological formation. At the time that the data is beingcollected at the hydrophones, there is no existing knowledge as to theextent in area of the subsurface formation (as defined by its x and yco-ordinates) or the depth z of the subsurface interfaces at whichseismic energy is reflected. Mathematical operations based on theacoustic wave equation above are used to “migrate” the signals collectedby the hydrophones to their x, y and depth co-ordinates of thesubsurface reflection points. All of these “migration” algorithmsrequire stable and consistent spatial sampling of the measured wavefield in order to accurately reconstruct the correct position, depth andimportantly the amplitude and phase of the signal which may get usedlater in the upstream flow for hydrocarbon prediction.

The use of one streamer towed along a single linear sail line at a timecollects a limited set of what is referred to as a “2-D in-line seismicdata”, which is a useful and relatively inexpensive way of conducting amarine seismic survey. When a single streamer is towed along a singlesail line, cross-line data is not acquired and the data set has anazimuth of essentially 0±10 degrees which is the industry acceptedlimited of feather tolerated when acquiring 2D in-line data using themethods of the prior art. These signals received by the hydrophones canbe collated together in what is termed a “gather” by collecting thesource-hydrophone pairs that share a CMP. The number ofsource/hydrophone pairs that make up a gather is subsequently termed the“fold” of the gather.

A “3-D seismic data set” is generated when multiple streamers are towedin parallel along a single linear sail line. It is not unusual for thestreamers to be spaced up to 100 meters apart and be 6000 meters long.The number of streamers and the size of the area being surveyeddetermine to a large degree the cost of a seismic survey. The size ofthe vessels required to tow these long streamers over vast areas ofocean also contribute substantially to the cost of the survey. Due tothe total number of sail lines required to build coverage of an area ofinterest, it is generally cheaper and therefore more desirable to usethe prior art 3D acquisition methods than the prior art 2D acquisitionmethods described above. By way of example, assuming that the area beingsurveys is 50 km wide and 20 km across and using the 3D streamer arrayof FIG. 8, the full survey area can be traverse using 80 parallel saillines at a distance of 250 meters apart. To collect the same density ofdata using an equivalent prior art 2D seismic acquisition arrangementwould require 400 sail lines to be traversed. This gives a cross-linebin dimension of 50 meters.

Using either 2D or 3D surveying, multiple parallel adjacent linear saillines are traversed so that the traversed ocean surface area overlaysthe subsurface area of interest. Using the methods of the prior art, thequality of the acquisition of seismic data relies to some extent on theskill of the towing vessel operator to accurately traverse thepredefined parallel adjacent linear sail line/s and their ability toensure that the orientation of each streamer is maintained parallel toand in line with the linear sail lines. When there are multiplestreamers as used for 3D seismic acquisition, that task is not only verydifficult but is also critical to the quality of the informationcollected. It is not uncommon to abandon a sail line part way throughbecause the streamers can not be kept parallel to the sail line due toloss or lack of control or strong currents and adverse weatherconditions and consequently great expense can be incurred because ofdelays or the need to redo all or part of a predefined sail line.

Methods have been proposed in which marine seismic data is acquired withthe streamer towing vessel following a circular sail line. For example,U.S. Pat. No. 4,486,863 (French) discloses a method wherein a streamertowing ship moves along circular paths with the streamer following thiscircular path. Each of the circles is offset along an advancing line.The towing ship completes a full circle and then leaves the completedcircle tangentially to move on from one circle to the next. However,there is a finite amount of curvature that can be put on a streamerresulting in a large track distance ratio (i.e. a large ratio betweenthe actual distance traversed by the vessel compared with the nominalsail-line distance). It is also very difficult to insert in-fill passesusing this style of acquisition. This is a very inefficient way tocollect 3D seismic data, and the additional time taken to acquire thedata equates to an increase in the cost of the acquisition.

U.S. Pat. No. 4,965,773 discloses a method of gathering and mappingseismic data of a marine region which contains a stationary bodycomprising the steps of defining a spiral path using a point on the bodyas the origin of the spiral, and towing a transmitter/receiver streameralong the spiral path to gather seismic data. The method is directed foruse in data collection around objects such as small islands, saltfingers present in the substratum of similar point-like structures. Inthe preferred embodiment, the radial distance between the spiral turnsis constant as given by an Archimedean spiral. This is also a veryinefficient way to collect 3D seismic data, and the additional timetaken to acquire the data equates to an increase in the cost of theacquisition.

US Patent Publication Number 2008/0285381 (Moldoveanu et al) describestowing a seismic spread including a single source and a plurality ofstreamers with all of the streamers being actively steered to maintaineach of the streamers on a generally curved advancing path. The radiusof the generally curved advancing path is described as being around5,500-7,000 m, resulting in a curved path having a circumference ofaround 34,000-44,000 m. Given an average streamer length of around 6,000m, it can immediately be seen the length of each streamer covers only asmall arc-length of the circular path being traversed. This method ofacquisition will inherently have only a small amount of deviation fromtraditional linear 3D acquisition systems with the added expense ofhaving to actively steer a plurality of streamers to avoid entanglementof the streamers during acquisition.

Methods have also been proposed in which marine seismic data is acquiredwith the streamer towing vessel following a linear sail line with thesource vessel traversing a non-linear path. By way of example, U.S. Pat.No. 3,806,863 (Tilley et al) discloses a system including a streamervessel traversing a linear sail line with one or more source vessels or“shooting boats” traversing a zigzag course. The shooting boat proceedsalong the firing leg from a position near to the base course of therecording boat to its sideways extreme position, or vice versa. Theseismic source of one of the shooting boats is fired at a repetitivetime interval as that shooting boat traverses a flanking zigzag courseline including a firing leg, or segment, oblique to the base course ofthe recording boat. This method of acquisition inherently demands theuse of more than one vessel which can result in unnecessary expense withlimited flow-on benefits. More significantly, controlling the zig-zagcourse of the shooting vessels is very difficult to achieve in practice,making this method prohibitively expensive.

U.S. Pat. No. 3,921,124 (Payton) describes a method of deriving a 3Dseismic dataset with a seismic vessel pulling a single streamer whichtraverses a linear sail line. At the same time, one or more remotecontrollable mobile seismic sources are actively steered along aperiodic path which facilitates the determination of common depth pointdata along a plurality of lines parallel to the line of survey, therebyproducing 3D seismic information. The positions of the sources aresystematically controlled using a “controlled paravane”. A seismicsource is a large piece of equipment that requires a fair amount offorce to deviate. To the best of the knowledge of the present inventors,such a controllable paravane has never actually been designed andemployed to date.

There remains a need in the art for an alternative marine seismic dataacquisition method and related system that produces a relatively cheap,multi-azimuth seismic dataset.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of acquiring marine seismic data using an acoustic source togenerate an acoustic signal, a portion of which is reflected at one ormore subsurface formation interfaces as a seismic signal, the methodcomprising:

-   -   a) sailing a surface vessel along a sail line which lies over a        survey area while towing a seismic streamer, the sail line        having a sinusoidal configuration defined by an amplitude and a        wavelength, the streamer including a plurality of hydrophones        for receiving the reflected portion of the acoustic signal,        characterised in that the streamer follows the sinusoidal        configuration of the sail line while seismic data is acquired,        the streamer having a length at least equal to the distance        travelled by the surface vessel as it sails along one full        wavelength of the configuration as measured along the sinusoidal        sail line.

In one form of the first aspect of the present invention the method ofacquiring marine seismic data further comprises:

-   -   b) dividing a survey area using a grid to form a plurality of        bins;    -   c) collating the seismic signals using the plurality of bins;        and    -   d) repeating step a) to populate each bin with seismic data,        wherein a range of offsets associated with each event varies        between adjacent cross-line and in-line bins.

According to a second aspect of the present invention there is provideda method of undertaking a seismic survey over a geological structurewithin a survey area, the method comprising the steps of:

-   -   a) transmitting an acoustic source signal from a source;    -   b) measuring a response signal at each of a plurality of        hydrophones arrayed in a streamer in the survey area, the        response signal being indicative of an interaction between the        source signal and the geological structure;    -   c) logging the orientation and position of the source relative        to the plurality of hydrophones; and;    -   d) gathering a plurality of response signals for a range of        source/hydrophone pairs to provide a survey data set,    -   the method characterised in that the plurality of hydrophones        are arranged in a sinusoidal configuration having an amplitude        and a wavelength relative to a nominal linear sail line whereby        the survey data set includes a variable offset range in both the        in-line and cross-line directions

In one form, using the method of the second aspect of the presentinvention, the streamer has a length at least equal to the distancetravelled by the surface vessel as it sails along one full wavelength ofthe configuration as measured along the sinusoidal sail line. Thestreamer length can be longer than this if desired. In one form of thefirst or second aspect of the present invention, the source is generatedfrom the surface vessel such that the both the source and the pluralityof hydrophones traverse a sinusoidal sail line.

In one form of the first or second aspect of the present invention, thestreamer is one of a plurality of streamers being towed along asinusoidal sail line by a surface vessel and wherein each streamer isseparated from each neighbouring streamer by a distance in the range of100 to 400 m. In one form of the first or second aspect of the presentinvention, the amplitude is one of: in the range of 200 to 1600 meters,in the range of 800 to 1600 meters, or in the range of 400 to 1200meters. In one form of the first or second aspect of the presentinvention, one or both of the wavelength and amplitude is uniform duringeach pass over the survey area.

In one form of the first or second aspect of the present invention, thesurface vessel completes a first or a previous pass over the survey areaand steps a) to d) are repeated as the surface vessel completes a secondor subsequent pass over the survey area. In one form of the first orsecond aspect of the present invention, a second or subsequent pass isstaggered from a first or a previous pass along the length of the surveyarea by a distance equal to the amplitude of the sinusoidalconfiguration of the first or the previous pass. Alternatively oradditionally, a second or subsequent pass is staggered from a first or aprevious pass across the width of the survey area by one quarter of thewavelength of the sinusoidal configuration.

In one form of the first or second aspect of the present invention, asecond or subsequent pass across the survey area is acquired with thecenter line of the sinusoidal configuration of the second or subsequentpass being arranged at an angle to the center line of a first or aprevious pass across the survey area. The angle may vary but for ease ofprocessing, the angle is preferably selected from the group consistingof: 30, 45, 60 or 90 degrees.

In one form of the first or second aspect of the present invention, atleast three passes across the survey area are performed with the centerline of each of the at least three pass being arranged at 60 degrees tothe center line of another of the at least three passes. In another formof the first or second aspect of the present invention, at least twopasses across the survey area are performed with the center line of eachof the at least two pass being arranged at 90 degrees to the center lineof other of the at least two passes.

In one form of the first or second aspect of the present invention, thesource is one of a plurality of sources, and wherein one of theplurality of sources transmits a signal from a surface vessel travellingalong a nominal linear sail line and another of the plurality of sourcestransmits a signal from a surface vessel travelling along a sinusoidalsail line.

In one form of the first or second aspect of the present invention, thesource vessel is one of a first source vessel and a second sourcevessel, and wherein the second source vessel tracks a cosine of the pathtravelled by the first source vessel such that the sinusoidal sail lineof the second source vessel is half a wavelength out of phase with thesinusoidal configuration of the first source vessel. In this form, asecond or subsequent pass may be staggered from a first or a previouspass along the length of the survey area by a distance equal to twicethe amplitude of the sinusoidal sail line travelled by the first orsecond source vessel during the first pass or a previous pass.

In one form of the first or second aspect of the present invention, asecond or subsequent pass across the survey area may be acquired using aplurality of source vessels, with a second source vessel tracking asinusoidal sail line having its center line arranged at an angle to thecenter line of the sinusoidal sail line tracked by a first sourcevessel. The angle may be selected from the group consisting of: 30, 45,60 or 90 degrees. Advantageously, the first or second source vessel mayalso be the surface vessel.

According to a third aspect of the present invention there is provided amethod of planning a survey of an area that is thought or known tocontain a subterranean hydrocarbon bearing reservoir, comprising:

-   -   creating a model of the area to be surveyed including a        seafloor, a rock formation containing a postulated hydrocarbon        bearing reservoir beneath the seafloor, and a body of water        above the seafloor;    -   setting values for depth below the seafloor of the postulated        hydrocarbon reservoir and material properties of the geological        structure; and    -   performing a simulation of the method of undertaking a survey of        any form of the first aspect of the present invention.

According to a fourth aspect of the present invention there is provideda method of planning a survey of an area that is thought or known tocontain a subterranean hydrocarbon bearing reservoir, comprising:

-   -   creating a model of the area to be surveyed including a        seafloor, a rock formation containing a postulated hydrocarbon        bearing reservoir beneath the seafloor, and a body of water        above the seafloor;    -   setting values for depth below the seafloor of the postulated        hydrocarbon reservoir and material properties of the geological        structure; and    -   performing a simulation of the method of acquiring seismic data        of any form of the second aspect of the present invention.

According to a fifth aspect of the present invention there is provided aset of survey data acquired using the method of any one form of thefirst or second aspects of the present invention.

According to a sixth aspect of the present invention there is provided amethod of storing and utilizing marine survey data comprising:

-   -   obtaining a set of survey data according to the fifth aspect of        the present invention; and    -   analyzing the set of survey data to obtain information relating        to a geological structure underlying a body of water.

According to a seventh aspect of the present invention there is provideda method of acquiring marine seismic data substantially as hereindescribed with reference to and as illustrated in the accompanyingillustrations. According to an eighth aspect of the present inventionthere is provided a method of undertaking a seismic survey over ageological structure within a survey area substantially as hereindescribed with reference to and as illustrated in the accompanyingillustrations.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the objectives of the invention and other desirablecharacteristics can be obtained is explained in the followingdescription and attached figures in which:

FIG. 1 schematically illustrates a source signal being transmitted froma source and a response signal being measured at each of a plurality ofreceivers arrayed in a survey area, the response signal being indicativeof an interaction between the source signal and the geologicalstructure;

FIG. 2 illustrates a sinusoidal sail line and the length of the streameras well as the amplitude and frequency of the sinusoidal configurationrelative to a nominal linear sail line;

FIGS. 3 a to 3 d illustrate the seismic coverage lines for a pluralityof shots as the streamer towing vessel traverses the sinusoidal sailline of FIG. 2;

FIG. 4 illustrates the event data collected in five adjacent cross-linebins whilst the streamer towing vessel of FIG. 10 follows a sinusoidalsail line;

FIG. 5 illustrates the event data collected in five adjacent in-linebins whilst the streamer towing vessel of FIG. 10 follows a sinusoidalsail line;

FIG. 6 illustrates a top view of a survey area showing the acquisitioncoverage for a first pass across the survey area;

FIG. 7 illustrates a top view of a survey area showing the acquisitioncoverage for a three passes across the survey area;

FIGS. 8 a and 8 b is a side-by-side comparison of a Rose diagram for aconventional 3D multi-streamer configuration and a Rose diagram for oneembodiment of the method of the present invention;

FIGS. 9 a and 9 b is a side-by-side comparison of the azimuthaldistribution for a conventional 3D multi-streamer configuration and theazimuthal distribution for one embodiment of the method of the presentinvention;

FIGS. 10 a to 10 e illustrates various streamer-source arrangements foracquiring data using various embodiments of the method of the presentinvention;

FIG. 11 illustrates an alternative embodiment of the present inventionusing a first and second source vessel, the second source vesseltracking a cosine of the path travelled by the first source vessel suchthat the sinusoidal sail line of the second source vessel is half awavelength out of phase with the sinusoidal sail line of the firstsource vessel; and,

FIGS. 12 a and 12 b illustrate an alternative embodiment of the presentinvention using a plurality of source vessels with a second sourcevessel tracking a path arranged at an angle to the path travelled by thefirst source vessel during acquisition.

It is to be noted that the figures are not to scale and illustrate onlytypical embodiments of this invention, and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments. Like reference numerals refer to likeparts.

DETAILED DESCRIPTION

Particular embodiments of the present invention are now described.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these details and thatnumerous variations or modifications from the described embodiments maybe possible. It will be understood that the invention is equallyapplicable to surveying in freshwater, for example large lakes orestuaries, as it is to seawater. Thus references below to the seabedshould not be regarded as limiting and should be interpreted as coveringa lakebed, riverbed or equivalent. The terminology used herein is forthe purpose of describing particular embodiments only, and is notintended to limit the scope of the present invention. Unless definedotherwise, all technical and scientific terms used herein have the samemeanings as commonly understood by one of ordinary skill in the art towhich this invention belongs.

The term “sinusoidal” as used throughout this specification refers to asmoothly varying periodic oscillating waveform which has a fundamentalshape expressed by the equation y=A sin x, where x is an angle measuredin degrees and A is the amplitude of the wave. It should be noted that acosine wave can also be considered to be “sinusoidal” becausecos(x)=sin(x+π/2).

Using the method of acquiring marine seismic data of the presentinvention, an acoustic source is used to generate an acoustic signal, aportion of which is reflected at one or more subsurface formationinterfaces as a seismic signal. The method comprises sailing a surfacevessel along a sail line which lies over a survey area while towing aseismic streamer, the sail line having a sinusoidal configurationdefined by an amplitude and a wavelength, the streamer including aplurality of hydrophones for receiving the reflected portion of theacoustic signal, characterised in that the streamer passively followsthe sinusoidal configuration of the sail line while seismic data isacquired, the streamer having a length at least equal to the distancetravelled by the surface vessel as it sails along one full wavelength ofthe configuration as measured along the sinusoidal sail line.

Particular embodiments of the present invention are now described withreference to FIGS. 1 to 11 in which a seismic survey is undertaken overa geological structure within a survey area. With reference to FIG. 1,the method comprises the steps of a) transmitting a source signal (110)from an acoustic source (112); b) measuring a response signal (114) ateach of a plurality of receivers or hydrophones (116) arrayed in thesurvey area (118), the response signal (114) being indicative of aninteraction between the source signal (110) and a geological structure(120); c) logging the orientation and position of the source (112)relative to each of the plurality of hydrophones (116); and; d)gathering a plurality of response signals (114) for a range ofsource/receiver pairs to provide a survey data set. The seismic responsesignal is measured at each of a plurality of hydrophones arranged atspaced apart intervals along the length of the single seismic streamer(120). The seismic response signal could equally be acquired using aplurality of hydrophones and/or geophones arranged within an oceanbottom cable resting on the seafloor. In the embodiment illustrated inFIGS. 2 to 7, a single acoustic source (112) is used along with a singlestreamer (120), the acoustic source (112) travelling with the vessel(114) that is used to tow the single streamer (120). Othersource/streamer configurations are described below with reference toFIGS. 10 a to 10 e.

With particular reference to FIGS. 2 and 3, a streamer-towing vessel(114) is shown traversing a sinusoidal sail line (116) lies over asurvey area (118) whilst towing a single seismic streamer (120). Thus,the method and system of the present invention differs fundamentallyfrom those of the prior art in that, instead of following a nominallinear sail line (126) as it crosses the survey area (118), thestreamer-towing vessel (114) follows the sinusoidal sail line (116).Using the method of present invention the source (112) and the pluralityof hydrophones (116) are arranged in a sinusoidal configuration relativeto a nominal linear sail line whereby the survey data set includes avariable offset range in both the in-line and cross-line directions.

The speed and direction of the vessel (114) is set at suitable values toencourage the streamer (120) to follow the sinusoidal sail line (116)whilst seismic data is being acquired without the need to actively steerthe streamer. Traversing a sinusoidal sail line whilst passively towingthe streamer provides a number of benefits over the prior art. Firstly,the method of the present invention allows the use of current streamertechnology, as the stress and strain on the streamer is essentially thesame as that experienced by streamers performing conventional 3D or 2Dacquisition when the vessel turns at the end of a given pass across thesurvey area, in preparation for the next acquisition pass across thesurvey area. This overcomes any expense associated with developingspecialist streamers and devices for actively steering the streamers.Secondly, the seismic data set acquired will have a time and spacevariant azimuthal content which may help 3D subsurface information to beascertained, even whilst using only one streamer, as explained ingreater detail below.

For clarity purposes in FIG. 2, the sinusoidal sail line (116) isindicated as a dotted line with the seismic streamer (120) beingindicated as a solid line. The term “amplitude” as used throughout thisspecification refers to the distance from one extremity (121) of anoscillation of a sine wave to the middle point or center line (123) ofthe sine wave. The term “frequency” refers to the number of oscillationsof a wave per unit time. The frequency thus represents the rate at whichthe fundamental shape repeats itself. The term “wavelength” refers tothe distance, measured in the direction of propagation of a wave,between two successive points (125 and 127, respectively) that arecharacterised by the same phase of oscillation. Each streamer (120) hasa first proximal end (122) and a second distal end (124), the firstproximal end (122) being that end of the streamer (120) that is locatedclosest to the vessel (114). Using the methods of the present invention,the length of the seismic streamer (120) measured as the distancebetween the first proximal end (122) and the second distal end (124) isat least equal to the distance travelled by the surface vessel as itsails along one full wavelength of the configuration as measured alongthe sinusoidal sail line (116).

The common mid points of the various source/hydrophone pairs are plottedin a manner analogous to that described above for prior art seismicacquisition methods. As a consequence of causing the streamer (120) tofollow the sinusoidal path of the sail line (116), seismic data signalsare collected at common mid points which fall along surface mid-pointcoverage line (128) for each shot from the source. One such coverageline (128) is shown in FIG. 2 for a first shot. As a plurality ofsuccessive shots are fired, a corresponding configuration of coveragelines (128) are generated, with ten such coverage lines (128)illustrated in FIG. 3 a, twenty such coverage lines (128) illustrated inFIG. 3 b, thirty such coverage lines (128) illustrated in FIG. 3 c andforty such coverage lines (128) illustrated in FIG. 3 d. As can be seenfrom FIGS. 3 a to 3 d, each successive coverage line (128) is offsetfrom each preceding coverage line (128) due to the fact that the vessel(114) continues to move along the sinusoidal sail line (116) betweensuccessive shots.

With reference to FIG. 4, the shaded acquisition areas (130) representthe area over which seismic signals are collected as the streamertravels along a sinusoidal sail line path (116) during a single or firstpass (174) across the survey area (118). The area (130) represents thecombined effect of all of these surface mid-point coverage lines (128).The survey area (118) is divided into a plurality of bins (132) using agrid in a manner analogous to that described above for prior art seismicacquisition methods. Depending on the amplitude and frequency of thesinusoidal sail line (116), the survey area (118) covered using themethod of the present invention is comparable to the area covered usingthe prior art 3D multiple streamer configurations. The offset range isdefined by the absolute difference between the minimum and maximumoffset present in a bin gather. Using conventional acquisition, everybin (132) is populated with a full complement of offsets ranging fromthe near offset to the far offset. By comparison, the method and processof the present invention produces a variable offset range in both thein-line and cross-line direction.

The breakout portion below the sail line pictorial in FIG. 4 illustratesthe event data collected in five adjacent cross-line bins (134), (136),(138), (140) and (142) as a function of offset on the x-axis versus timeon the y-axis. The breakout portion below the sail line pictorial inFIG. 5 illustrates the event data collected in five adjacent in-linebins (144), (146), (148), (150) and (152) as a function of offset on thex-axis versus time on the y-axis. In each of the five adjacentcross-line bins (134), (136), (138), (140) and (142), a correspondingevent is depicted using a plurality of solid lines (154), (156), (158),(160) and (162) to represent the partial offsets recorded in each binand a dotted line which represents the potential full range of offsetsthat would have been measured using prior art 3D multi-streamer linearacquisition methods. It can be seen from FIG. 4, that there aredifferent offsets associated with a single event in the adjacentcross-line bins (134), (136), (138), (140) and (142). Similarly, in eachof the adjacent in-line bins, (144), (146), (148), (150) and (152), acorresponding event is depicted using a plurality of solid lines (164),(166), (168), (170) and (172) to represent the partial offsets recordedin each bin and a dotted line which represents the potential full rangeof offsets that would have been measured using prior art 3D linearmulti-streamer acquisition methods. It can be seen from FIG. 5, thatthere are different offsets associated with a single event in theadjacent in-line bins (144), (146), (148), (150) and (152) as comparedto the offsets associated with the same event in the adjacent cross-linebins (134), (136), (138), (140) and (142).

As best seen in FIGS. 3 a to 3 d, the streamer location varies inposition and time is such as way that at each successive shot, the datais being acquired over a different range of common mid-points. Moreover,the streamer location deviates from the center line of the sinusoidalsail path (116) over time, resulting in a variation in cross-line andin-line offsets with time.

The offset range in each bin (132) using the method of the presentinvention will depend on such relevant factors as the frequency of theshots, the number and location of acoustic sources (112) used, thenumber and distribution of hydrophones along the length of the streamer(120), the sinuosity of the sail line, and the number and arrangement ofstreamers used. By way of example, the streamer can be 3 km long with120 hydrophones spaced at intervals of 25 m along the length of thestreamer. It is to be understood that the length of the streamer canvary between 3 km to 8 km. Generally speaking, the longer the streamer,the greater the number of hydrophones are available for collecting dataand the greater the fold in the data. However, longer streamers resultin longer offsets which can make it more difficult to process the datathat is acquired. Consequently, a balance needs to be sought, with bestresults achieved using a streamer length in the range of 4.5 to 5 km forsome types of hydrocarbon exploration or development objectives.

The fold, azimuth and coverage using the method of the present inventionare all dependant on a number of relevant variables, the main ones beingrelated to the level of sinuosity of the sail line (116) relative to thelength of the streamer (120). The sinuosity is set by the amplitude,wavelength and frequency of the sinusoidal sail line (116) during anygiven pass across the survey area (118). For consistency of results, theamplitude, wavelength and frequency of the sinusoidal sail line is keptuniform during a first or previous pass (174) and during a second orsubsequent pass (178) across the survey area during acquisition asillustrated in FIGS. 6 and 7. When this is done, the velocity of thevessel (114) is adjusted to aid in keeping the amplitude and wavelengthof the sinusoidal sail line (116) as uniform as possible and in thisregard, the firing of shots is timed as a function of the changingposition of the streamer (120) over time rather than following a regularshot firing schedule. It is to be understood that it is possible to varyone or all of the amplitude, wavelength and frequency along the sailline (116) but that this will make processing the data acquired morecomplicated. Acquisition can commence at any point in the sine wave,there being no requirement that a given pass across the survey area(118) start or end at the center line or at a peak of the sine wave,although acquiring data in this way may make it easier to process lateron.

It is readily apparent from FIGS. 6 and 7, that the greater theamplitude of the sinusoidal sail line (116), the greater the coverage ina given data set per pass due to an increase in the acquisition area(130). However, there are limits on the degree to which the streamer(120) can be flexed which are dependent in part on the minimum radius ofcurvature at an inflection point which can be achieved for a givenstreamer design. Moreover, if the amplitude is too high, there is littledifference between the method of the present invention and theconventional linear acquisition methods of the prior art. The bestcompromise is achieved using a sinusoidal sail line (116) having anamplitude in the range of 400 to 200 meters, with the range of 800 to1600 meters being preferred.

The “fold” is defined by the number of traces with a common mid-pointwhere that mid-point is exactly half the distance between asource/receiver pair. Conventional acquisition is designed to create avery regular and relatively high fold. In contrast, the method andprocess of the present invention includes some areas of higher fold thanother areas with the overall fold being far lower than that ofconventional 3D acquisition techniques. However, using the process ofthe present invention makes it possible to complete spatial coverage fora given bin density as would be achieved using conventionalmulti-streamer 3D acquisition techniques using a single passively towedstreamer instead of having to tow a multi-streamer array or having toactively steer one or more streamers.

After a first pass (174) across the survey area (118) has beencompleted, a second pass (176) and subsequent passes (178) can be madeto complete the acquisition, the number of subsequent passes (178) beingdependent on the size of the survey area (118) relative to the amplitudeof the sine wave of the sail line (116) during the first pass (174) or aprevious pass. It is to be understood that acquisition can be continuedas the vessel (114) is completing a turn to commence a subsequent pass(178). With reference to FIGS. 6 and 7, the survey area (118) is shownin top view as a rectangular planar area having a width defined by thefirst side (179) and the second side (180) of a rectangle, and a lengthdefined by the third side (182) and fourth side (184) of the rectangle.During each pass (174, 176 or 178) across the survey area (118), thevessel (114) travels across the width of the survey area (118) from thefirst side (179) to the second side (180) following a sinusoidal sailline (116), the center line of which is a linear nominal sail line(126). Looking at a completed first pass (174) across the survey area(116), seismic data is collected over the shaded area (130) in FIG. 7for that pass.

As illustrated in FIG. 7, the second pass (176) and each subsequent pass(178) is staggered from the first pass (174) or a previous pass alongthe length of the survey area (118) by a distance equal to the amplitudeof the sinusoidal sail line (116) travelled by the vessel (114) duringthe first pass (174) or a previous pass. The second pass (176) and eachsubsequent pass (178) is staggered from the first pass (174) or aprevious pass across the width of the survey area (118) by one quarterof the wavelength of the sinusoidal sail line (116). This arrangement ofpasses represents one way of achieving good coverage across a surveyarea. Other arrangements are illustrated in FIGS. 10 a to 10 e. It isapparent from FIG. 7 that when the shaded area (130) being acquired fora given pass (174) overlaps with the shaded area being surveyed for asubsequent pass (178), the bins which fall in the overlappingacquisition area gather data from the same event but for a differentpartial range of offsets.

The “azimuth” is the angle of a line defined by the source and receivercoordinates of a measured seismic trace measured in degree clockwisefrom the North. A Rose diagram describes the overall azimuthaldistribution of an acquisition configuration design. The distance fromthe center describes the offset whilst the angle from North defines theazimuth FIG. 8 a shows a Rose diagram of a conventional 3D acquisitionprocess whilst FIG. 8 b shows a Rose diagram using the method andprocess of the present invention after the vessel (114) has travelledone wavelength along the sinusoidal sail line (116). FIGS. 9 a and 9 bshows a side-by-side comparison of the percentile distribution ofseismic data as a function of specific azimuths. FIG. 9 a shows thepercentile distribution for conventional acquisition which shows peaksin the nominal acquisition direction. FIG. 9 b shows the percentiledistribution for the process of the present invention showing how thedata is scattered more evenly over a broader span of azimuths.

Each of FIGS. 10 a-e depicts alternative embodiments of the presentinvention. In these embodiments, each towing vessel is shown as symbol114 and the approximate location of the acoustic source or ‘shot gun’ isdepicted as symbol 112. The nominal sail line direction per pass (126)is illustrated using dotted and dashed lines with the sinusoidal sailline (116) being illustrated as a solid line with an arrow at one end toindicate the direction of travel of the vessel (114). The streamers arenot shown in FIGS. 10 a-e. Clearly, all the configurations illustratedin FIGS. 10 a-e have different economic costs but it is however anoperator decision to choose to use a more expensive towing configurationif the task is completed more quickly or the increased usefulness of theresults justifies the cost differential.

FIG. 10 a shows a variation on the configuration of FIG. 3 at least twoacoustic sources (112) providing additional synchronised shots toachieve a wide-azimuth acquisition. In the embodiment illustrated inFIG. 10 a, one of the two acoustic sources being fired from the streamertowing vessel (114) and the other of the two acoustic sources beingfired from a gun boat (190) which follows being the streamer towingvessel (114) along the same sinusoidal sail line (116). The use of asecond acoustic source is advantageous as it creates acoustic signalsthat reinforce subsurface features or cancel out noise and otherspurious signals. Additional acoustic sources also provide signals froma variety of directions and their use can create information ofsubsurface formations that would not be detectable if only one acousticsource is used. Generally speaking, gun boats are much cheaper to hirethan a vessel that is capable of towing a streamer, so the costs can becontained and additional benefits can be derived at the expense oflonger and more complicated signal processing.

FIG. 10 b illustrates an alternative configuration in which at least twoacoustic sources (112) are being used to achieve a wide-azimuthacquisition, again using additional synchronised shots. In thisembodiment, the gun boat (190) travels along a nominal linear sail line(192) which is parallel to but offset from the center line (126) of thesinusoidal sail line (116) being followed by the streamer-towing vessel(114). The nominal linear sail line (192) being followed by the gunboat(190) may be on either side of the center line (126) of the sinusoidalsail line (116) being followed by the streamer towing vessel (114). Whenusing this configuration, the source vessel (190) is sailed in such amanner as to ensure that the gun boat (190) maintains a safe workingdistance (of at least 50 meters) from the streamer (120) at all timesduring the acquisition. FIG. 10 c illustrates yet another alternativeconfiguration similar to that illustrated in FIG. 10 b using a pluralityof streamers (120) to achieve a wide-azimuth acquisition using multiplestreamers and multiple sources whilst the towing vessel (114) follows asinusoidal sail line (116). In this embodiment, each streamer isseparated from each neighbouring streamer by a nominal distance, forexample 100 to 400 m to minimize the potential for tangling of thestreamer as the vessel (114) turns around at the end of a completed passto prepare for the next pass across the survey area.

The second or any subsequent pass (176 or 178, respectively) across thesurvey area (118) can be acquired with the center line (123) of thesinusoidal sail line (116) being arranged at an angle to the center lineof a previous sinusoidal sail line. This angle can be any value but 30,45, 60 or 90 degrees are preferred for ease of processing. By way ofexample, three passes can be performed with the center line of each passbeing arranged at 60 degrees to the center line of the preceding pass asillustrated in FIG. 10 d for a vessel (114) towing a single streamer(120) to achieve a multi-azimuth acquisition. In the embodimentillustrated in FIG. 10 e, two passes across the survey area areperformed with the center line of each pass being arranged at 90 degreesto the center line of the preceding pass using a vessel (114) towing aplurality of streamers (120) to achieve a rich azimuth acquisition.

In another alternative embodiment illustrated in FIG. 11, two sourcevessels are used, with a second source vessel (180) tracking in frontof, or preferably, at the back of a first source vessel (182). In thisembodiment, the second source vessel tracks a “reverse sinusoid” orcosine of the path travelled by the first source vessel such that thesinusoidal sail line of the second source vessel is half a wavelengthout of phase with the sinusoidal sail line of the first source vessel.Using this arrangement allow more complete coverage to be achievedwithout needing to stagger subsequent passes to complete the acquisitionas described above in relation to FIG. 7. Using the arrangement of FIG.11, the second pass (176) and each subsequent pass (178) is staggeredfrom the first pass (174) or a previous pass along the length of thesurvey area (118) by a distance equal to twice the amplitude of thesinusoidal sail line (116) travelled by the surface vessel (114) duringthe first pass (174) or a previous pass.

With reference to FIGS. 12 a and 12 b, the second or any subsequent pass(176 or 178, respectively) across the survey area (118) can be acquiredusing a plurality of source vessels, with a second source vessel (180)tracking a sinusoidal sail line (184) having its center line (123)arranged at an angle to the center line (186) of the sinusoidal sailline tracked by the first source vessel (182). This angle can be anyvalue but 30, 45, 60 or 90 degrees are preferred for ease of processing.By way of example, two passes can be performed with the second sourcevessel (180) tracking a sinusoidal sail line (184) having its centerline (123) arranged at an angle of 60 degrees to the center line (186)of the sinusoidal sail line tracked by the first source vessel (182) asshown in FIG. 12 a or arranged at an angle of 90 degrees as shown inFIG. 12 b to achieve a multi-azimuth acquisition using two sourcevessels. In these embodiments, the first or second source vessel (180 or182, respectively) also serves the function of being a surface vessel(114) towing one or more streamer (not shown in FIG. 12 a or 12 b forclarity).

A key advantage of the present invention is that a single streamer canbe towed (as opposed to the more complex and expensive 3D multi-streamerarrays of the prior art) behind a vessel and the data acquired using themethod of the present invention can be used to create a 3-dimensionalstructural representation of the subsurface formations due to a theoffsets having both an in-line component and a cross-line component.When a single streamer is used, less information is collected using themethod of the present invention compared with conducting a conventionprior art 3D survey but the costs associated with hiring the larger andmore expensive vessels required to tow large 3D multi-streamer arrays isavoided. Another distinct advantage is the ability to cover a surveyarea in a short time thus resulting in a far smaller health, safety andenvironmental (HSE) imprint. It is however to be understood that aplurality of streamers can be used to acquire seismic data using theprocess of the present invention instead of using a single streamer,with the benefit of acquiring more data having to be weighed up againstthe additional cost and noise generated when towing a plurality ofstreamers.

The method and system of the present invention when used for seismicdata acquisition provides a data set that is more useful than an in-line2-D seismic survey approach but less complete than a data set acquiredconducting a convention 3-D seismic survey. Depending on the particulararrangement selected, the cost of using the marine seismic acquisitionmethod and system of the present invention is comparable to the 2-Dseismic survey approach and not as expensive and the time consuming asconducting a 3-D seismic survey. The low fold seismic dataset acquiredusing the method of present invention is particular useful for largescale reconnaissance purposes.

Although only a few embodiments of this invention have been described indetail above, those skilled in the art will readily appreciate that manymodifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of this invention.Alternatively, the ocean-bottom cable can be laid in a sinusoidalconfiguration whilst the source vessel sails along a sinusoidal sailline half a wavelength out of phase with the sinusoidal configuration ofthe ocean-bottom cable. Accordingly, all such modifications are intendedto be included within the scope of this invention.

What is claimed:
 1. A method of acquiring marine seismic data using anacoustic source to generate an acoustic signal, a portion of which isreflected at one or more subsurface formation interfaces as a seismicsignal, the method comprising: a) sailing a surface vessel along asinusoidal sail line which lies over a survey area while towing aseismic streamer, the sinusoidal sail line having a configurationdefined by an amplitude and a wavelength, the streamer including aplurality of hydrophones for receiving the reflected portion of theacoustic signal, wherein the streamer follows the configuration of thesinusoidal sail line while seismic data is acquired, and the streamerhas a length at least equal to the distance travelled by the surfacevessel as it sails along one full wavelength of the configuration asmeasured along the sinusoidal sail line.
 2. The method of acquiringmarine seismic data of claim 1, further comprising: b) dividing a surveyarea using a grid to form a plurality of bins; c) collating the seismicsignals using the plurality of bins; and d) repeating step a) topopulate each bin with seismic data, wherein a range of offsetsassociated with each event varies between adjacent cross-line andin-line bins.
 3. The method of claim 1, wherein the source is generatedfrom the surface vessel.
 4. The method of claim 1, wherein the streameris one of a plurality of streamers being towed along a sinusoidal sailline by a surface vessel, and each streamer is separated from eachneighbouring streamer by a distance in the range of 100 to 400 m.
 5. Themethod of claim 1, wherein the amplitude is one of: in the range of 200to 1600 meters, in the range of 800 to 1600 meters, and in the range of400 to 1200 meters.
 6. The method of claim 1, wherein at least one ofthe wavelength and amplitude is uniform during each pass over the surveyarea.
 7. The method of claim 2, wherein the surface vessel completes afirst or a previous pass over the survey area and steps a) to d) arerepeated as the surface vessel completes a second or subsequent passover the survey area.
 8. The method of claim 7, wherein a second orsubsequent pass is staggered from a first or a previous pass along thelength of the survey area by a distance equal to the amplitude of thesinusoidal configuration of the first or the previous pass.
 9. Themethod of claim 7, wherein a second or subsequent pass is staggered froma first or a previous pass across the width of the survey area by onequarter of the wavelength of the sinusoidal configuration.
 10. Themethod of claim 1, wherein a second or subsequent pass across the surveyarea is acquired with the center line of the sinusoidal configuration ofthe second or subsequent pass being arranged at an angle to the centerline of a first or a previous pass across the survey area.
 11. Themethod of claim 10, wherein the angle is selected from the groupconsisting of 30, 45, 60 and 90 degrees.
 12. The method of claim 1,wherein at least three passes across the survey area are performed withthe center line of each of the at least three pass being arranged at 60degrees to the center line of another of the at least three passes. 13.The method of claim 1, wherein at least two passes across the surveyarea are performed with the center line of each of the at least two passbeing arranged at 90 degrees to the center line of other of the at leasttwo passes.
 14. The method of claim 1, wherein the source is one of aplurality of sources, and one of the plurality of sources transmits asignal from a surface vessel travelling along a nominal linear sail lineand another of the plurality of sources transmits a signal from asurface vessel travelling along a sinusoidal sail line.
 15. The methodof claim 1, wherein the source vessel is one of a first source vesseland a second source vessel, and the second source vessel tracks a cosineof the path travelled by the first source vessel such that thesinusoidal sail line of the second source vessel is half a wavelengthout of phase with the sinusoidal configuration of the first sourcevessel.
 16. The method of claim 15, wherein a second or subsequent passis staggered from a first or a previous pass along the length of thesurvey area by a distance equal to twice the amplitude of the sinusoidalsail line travelled by the first or second source vessel during thefirst pass or a previous pass.
 17. The method of claim 15, wherein asecond or subsequent pass across the survey area is acquired using aplurality of source vessels, with a second source vessel tracking asinusoidal sail line having its center line arranged at an angle to thecenter line of the sinusoidal sail line tracked by the first sourcevessel.
 18. The method of claim 17, wherein the angle is selected fromthe group consisting of 30, 45, 60 and 90 degrees.
 19. The method ofclaim 17, wherein the first source vessel or the second source vessel isthe surface vessel.
 20. A method of undertaking a seismic survey over ageological structure within a survey area, the method comprising thesteps of: a) transmitting an acoustic source signal from a source; b)measuring a response signal at each of a plurality of hydrophonesarrayed in a streamer in the survey area, the response signal beingindicative of an interaction between the source signal and thegeological structure; c) logging the orientation and position of thesource relative to the plurality of hydrophones; and d) gathering aplurality of response signals for a range of source/hydrophone pairs toprovide a survey data set; wherein both the source and the plurality ofhydrophones are arranged in a sinusoidal configuration having anamplitude and a wavelength relative to a nominal linear sail line suchthat the survey data set includes a variable offset range in both thein-line and cross-line directions and the streamer has a length at leastequal to the distance travelled by the surface vessel as it sails alongone full wavelength of the configuration as measured along thesinusoidal sail line.
 21. The method of claim 20, wherein the source isgenerated from the surface vessel.
 22. The method of claim 20, whereinthe streamer is one of a plurality of streamers being towed along asinusoidal sail line by a surface vessel and wherein each streamer isseparated from each neighbouring streamer by a distance in the range of100 to 400 m.
 23. The method of claim 20 wherein the amplitude is oneof: in the range of 200 to 1600 meters, in the range of 800 to 1600meters, and in the range of 400 to 1200 meters.
 24. The method of claim20, wherein at least one of the wavelength and amplitude is uniformduring each pass over the survey area.
 25. The method of claim 20,wherein the surface vessel completes a first or a previous pass over thesurvey area and steps a) to d) are repeated as the surface vesselcompletes a second or subsequent pass over the survey area.
 26. Themethod of claim 20, wherein a second or subsequent pass is staggeredfrom a first or a previous pass along the length of the survey area by adistance equal to the amplitude of the sinusoidal configuration of thefirst or the previous pass.
 27. The method of claim 26, wherein a secondor subsequent pass is staggered from a first or a previous pass acrossthe width of the survey area by one quarter of the wavelength of thesinusoidal configuration.
 28. The method of claim 20, wherein a secondor subsequent pass across the survey area is acquired with the centerline of the sinusoidal configuration of the second or subsequent passbeing arranged at an angle to the center line of a first or a previouspass across the survey area.
 29. The method of claim 28, wherein thewherein the angle is selected from the group consisting of 30, 45, 60and 90 degrees.
 30. The method of claim 20, wherein at least threepasses across the survey area are performed with the center line of eachof the at least three passes being arranged at 60 degrees to the centerline of another of the at least three passes.
 31. The method of claim20, wherein at least two passes across the survey area are performedwith the center line of each of the at least two passes being arrangedat 90 degrees to the center line of other of the at least two passes.32. The method of claim 20, wherein the source is one of a plurality ofsources, and one of the plurality of sources transmits a signal from asurface vessel travelling along a nominal linear sail line and anotherof the plurality of sources transmits a signal from a surface vesseltravelling along a sinusoidal sail line.
 33. The method of claim 20,wherein the source vessel is one of a first source vessel and a secondsource vessel, and wherein the second source vessel tracks a cosine ofthe path travelled by the first source vessel such that the sinusoidalsail line of the second source vessel is half a wavelength out of phasewith the sinusoidal configuration of the first source vessel.
 34. Themethod of claim 33, wherein a second or subsequent pass is staggeredfrom a first or a previous pass along the length of the survey area by adistance equal to twice the amplitude of the sinusoidal sail linetravelled by the first or second source vessel during the first pass ora previous pass.
 35. The method of claim 20, wherein a second orsubsequent pass across the survey area is acquired using a plurality ofsource vessels, with a second source vessel tracking a sinusoidal sailline having its center line arranged at an angle to the center line ofthe sinusoidal sail line tracked by a first source vessel.
 36. Themethod of claim 35, wherein the angle is selected from the groupconsisting of 30, 45, 60 and 90 degrees.
 37. The method of claim 35,wherein the first or second source vessel is the surface vessel.
 38. Amethod of planning a survey of an area that is thought or known tocontain a subterranean hydrocarbon bearing reservoir, comprising:creating a model of the area to be surveyed including a seafloor, a rockformation containing a postulated hydrocarbon bearing reservoir beneaththe seafloor, and a body of water above the seafloor; setting values fordepth below the seafloor of the postulated hydrocarbon reservoir andmaterial properties of the geological structure; and performing asimulation of the method of undertaking a survey of claim
 1. 39. Amethod of planning a survey of an area that is thought or known tocontain a subterranean hydrocarbon bearing reservoir, comprising:creating a model of the area to be surveyed including a seafloor, a rockformation containing a postulated hydrocarbon bearing reservoir beneaththe seafloor, and a body of water above the seafloor; setting values fordepth below the seafloor of the postulated hydrocarbon reservoir andmaterial properties of the geological structure; and performing asimulation of the method of acquiring seismic data of claim 20.